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He added that he had a lot of confidence in the FBI and that they have a "difficult job" to do.

News of the scandal shocked officials in Washington in the immediate aftermath of the US election. Congressional leaders of both parties have said they should have been informed earlier.

'Prudent'

Democratic Senator Dianne Feinstein, who chairs the Senate Intelligence Committee, said Gen Petraeus, who stepped down on Friday, would give evidence only about the Benghazi attack, in which four Americans were killed.

He would not testify about his resignation over an affair with his biographer Paula Broadwell, she added.

Confirmation that Gen Petraeus will attend a committee hearing comes after days of reports focusing on the fallout from the revelation that he had an extramarital affair with Mrs Broadwell.

After the CIA director's resignation, it then emerged that Jill Kelley, who filed a harassment report over emails she received that led to the discovery of the affair, was in contact with Marine Gen John Allen.

The link to Gen Allen - currently commanding US forces in Afghanistan and nominated to be the top Nato commander in Europe - prompted the US defence secretary and the White House to stress they have complete confidence in him.

But Mr Panetta requested that Gen Allen's nomination as Supreme Allied Commander of US and Nato troops in Europe be placed on hold, saying it was the "prudent" thing to do.

Gen Allen has denied any wrongdoing and has spoken to Gen Martin Dempsey, chairman of the Joint Chiefs of Staff, to emphasise he is innocent of misconduct, a spokesman for Gen Dempsey said.

Divisive issue

Gen Petraeus could appear before congressional committees as early as Thursday, although no final date for a hearing has been confirmed.

Ms Feinstein, who chairs the Senate Intelligence Committee, said the general would be asked about the Benghazi attacks, which took place while he was in charge of the CIA.

The attack - initially blamed on a spontaneous protest over an anti-Islamic film - remains a divisive issue in Washington and a focal point of Republican anger.

Arizona Senator John McCain led a group of Republican senators on Wednesday calling for the establishment of a special committee to investigate the deaths of the four Americans, which included the Ambassador Christopher Stevens, in Benghazi.

They said there was need to find out whether there had been a cover-up over the attack, which Senator Lindsey Graham referred to in the same breath as Watergate and the Iran/Contra scandal.

Republicans have also criticised UN Ambassador Susan Rice, after she cited the protests as the reason for the attacks in appearances on US media.

On Wednesday, Mr Obama said Ms Rice had fulfilled her diplomatic duties with "skill and professionalism, and toughness and grace", and that "to besmirch her reputation was outrageous".

The president suggested that Republicans had criticised Ms Rice "apparently because they think she's an easy target", and said he would co-operate with congressional investigations into what happened in Benghazi.

Meanwhile, acting CIA Director Michael Morell began meetings with legislators in the Senate on Tuesday to answer questions about Gen Petraeus' resignation.

Legislators are concerned about a possible breach of national security after it was reported that classified material was discovered on Mrs Broadwell's computer.

However, FBI officials have said the agency concluded in its investigation that there had not been any security breach.

President Barack Obama has made his first public comments on the scandal at a news conference, before meeting business leaders.

His remarks concentrated on his economic agenda and plans for reaching a deal to avoid a package of spending cuts and tax rises known as the "fiscal cliff".

'Honorary consul'

While Gen Petraeus attempts to focus on Benghazi, Pentagon investigators are also examining as many as 30,000 pages of Gen Allen's documents.

They are said to include extensive communication between the general and Mrs Kelley, a 37-year-old, married Florida socialite.

The emails are now understood to have included pet names such as "sweetheart" or "dear" but were not sexually explicit or seductive, the Associated Press reports.

In May, Mrs Kelley reported anonymous, harassing emails to the FBI. The agency launched an investigation that traced the emails to Mrs Broadwell and revealed her affair with Gen Petraeus.

Mrs Kelley has been described as a "social liaison" for the MacDill Air Force Base near Tampa, Florida. Home to the US Central Command, both generals have served at the base in recent years.

The Kelleys are said to have developed close friendships with Gen Petraeus and Gen Allen, both of whom wrote letters supporting Mrs Kelley's twin sister, Natalie Khawam, in a messy custody battle for her son.

Mrs Kelley is known to have thrown extravagant parties for the officers posted there and in August was appointed as an honorary consul representing South Korea, ABC News reports.

It is reported that her Mercedes car displays consular licence plates reading "JK1".

In a recording of a phone call to police on Monday, Mrs Kelley asked for diplomatic immunity to have media crews waiting outside her house removed.

"I'm an honorary consul general, so I have inviolability, so they should not be able to cross my property. I don't know if you want to get diplomatic protection involved as well," Mrs Kelley can be heard saying.

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